1. Field of the Invention
The present invention relates to a method of manufacturing a GaAs metal semiconductor field effect transistor (GaAs MESFET), which method implants ions and diffuses silicon from a silicon thin film deposited on a surface of a GaAs compound semiconductor to dope silicon in an ohmic contact portion at a high concentration and which etches the contact portion by way of recess etching, thereby improving an electrical property of the transistor.
2. Description of related Art
Referring to FIGS. 1 (A) through (F), a process of manufacturing such a GaAs field effect transistor employing a self-aligned gate structure made of a thermostable metal (heatproof metal) is illustrated. As shown in FIG. 1 (A), a sensitive film 102 having a predetermined pattern is deposited over a GaAs substrate 101 and an impurity ion is implanted into the substrate 101 to form a n-channel impurity implanted layer 101a in the substrate 101. After the sensitive film 102 is removed, a tungsten silicide thin film 103 which is a thermostable metal thin film is formed on the substrate 101 by way of a sputtering method, as shown in FIG. 1 (B). Consequently, as shown in FIG. 1 (C), a sensitive film 102a of a predetermined pattern is formed on the film 103 and this film 103 is etched by a dry ion etching method using the sensitive film 102a as a mask to form a gate electrode 103a as shown in FIG. 1 (D).
Next, as shown in FIG. 1 (E), a sensitive film 102b is formed on the substrate 101 except on the impurity implanted layer 101a and an impurity of a high concentration is implanted to provide a highly concentrated n-type impurity layer 101b below the n-type impurity implanted layer 101a between the sensitive film 102b and the gate electrode 103a. Subsequently, as shown in FIG. 1 (F), the sensitive film 102b is removed and an ohmic contact 104 is then formed on the n-type impurity implanted layer 101a to complete a desired transistor.
By the way, the gate electrode 103a thus formed is substantially formed with a thermostable gate made of a tungsten compound such as tungsten silicide to prevent it from reacting with the GaAs substrate in a heat treatment such as an annealing. The ohmic contact 104 is typically formed by ion-implanting an impurity such as silicon at a high concentration with use of the gate electrode as a mask and activating the impurity by way of a subsequent heat treatment.
With such a conventional method, a thermostable alloy such as a tungsten silicide is employed as a gate electrode material because it prevents the substrate from the high temperature during the heat treatment and is unchanged in property. The gate, however, has a relatively large resistivity, thereby resulting in the problem that the transistor is reduced in operating speed. With such a conventional method, also, it is difficult to employ it to fabricate a high speed transistor or microwave transistor.
To reduce such a resistivity of the gate, a two-layered gate is employed. In the structure, when the gate is etched by dry etching, it is difficult to know an etching stop timing because the gate has a etching selection ratio different from the gate material.
Further, the substrate is roughened by the high concentrated impurity ion implantation executed to form the ohmic contact. For this reason, if electrical activation of the impurity is insufficient, then the resistance of the contact portion is increased, decreasing the electrical property of a channel area.